Self-oscillating helium magnetometer

ABSTRACT

A self-oscillating magnetometer having a metastable helium atom absorption cell optically pumped with the light emission line of 3888.65 Angstroms wavelength. The amount of light transmitted through the absorption cell at this wavelength is sensed by a photo-detector which has relatively high spectral response at the same wavelength. The photo-detector output signal provides a signal to a pair of Helmholtz coils for generating an RF resonance frequency field in the absorption cell and a signal to a display indicative of the ambient magnetic field strength.

United States Patent 1 Gasser III] 3,725,775

[ Apr. 3, 1973 [54] SELF-OSCILLATING HELIUM MAGNETOMETER [76] Inventor:Richard E. Gasser, 430 Elm Street,

Warminster, Pa. 18974 [22] Filed: Aug. 23, 1968 [21] Appl. N0.: 754,769

[52] US. Cl. ..324/0.5 R [51] Int. Cl. ..G0li' 33/00 [58] Field ofSearch ..324/0.5 R, 0.5 E, 0.5 F

[56] References Cited UNITED STATES PATENTS 3,122,702 2/1964 Franken..324/0.5 3,173,082 3/1965 Bell et al. ....324/0.5 3,350,632 10/1967Robinson ..324/0.5

OTHER PUBLICATIONS Bloom, Optical Pumping, reprinted from ScientificAmerican Oct. 1960.

Primary Examiner-Benjamin A. Borchelt Assistant Examiner-R. KinbergAttorneyEdgar J. Brower and Henry Hansen [57] ABSTRACT 7 Claims, 4Drawing Figures IGNITION CIRCUIT FILTER 17 I I9 2I 22 x LAMP EI CLTTMATCHING 1a NETWORK DISPLAY 2a VARIABLE 33 PHASE AMP.

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ABSORPTION CELL MATCHING NETWORK SELF-OSCILLATING I-IELIUM MAGNETOMETERThe invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor. BACKGROUND OFTHE INVENTION The present invention relates to optical pumpingmagnetometers, and more particularly to self-oscillating metastablehelium magnetometers for measuring weak magnetic fields.

Metastable helium magnetometers of the prior art, such as illustrated inU. S. Pat. No. 3,122,702 issued Feb. 25, 1964 to Peter A. Franken,generally operate with resonance radiation of approximately 10,830Angstroms wavelength which is the wavelength required to optically pumpthe metastable helium atoms to the second triplet P state (2 F).Photo-detectors presently available have relatively low spectralresponse at this wavelength resulting in an intolerably highnoise-tosignal ratio. A locked oscillation (or swept) system such asdisclosed in FIG. 4 of U. S. Pat. No. 3,256,500

issued June 14, 1966 to James T. Arnold is therefore required instead ofa more simple self-oscillation system such as illustrated in FIG. 5 ofthe same patent. This is because, when operating on the 10,830 Amgstromemission line, the output of the photo-detector is insufficient tosupport self-oscillation.

SUMMARY OF THE INVENTION Accordingly, it is a general purpose of thepresent invention to provide a metastable helium magnetometer operatingon a light emission line of a wavelength which produces a photo-detectoroutput signal of sufficient magnitude to sustain self-oscillation andthereby result in a magnetometer of reduced size and weight anddemanding less power to greatly simplified circuits while providingincreased serviceability, reliability and sensitivity at a substantialreduction in cost.

This is accomplished according to the invention by optically pumping themetastable helium absorption cell with the light emission line of3888.65 Angstroms wavelength and directly coupling the photo-detectoroutput to the RF resonance coils at the absorption cell.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of aself-oscillating metastable helium magnetometer constructed according tothe present invention;

FIG. 2 is a quantum energy level diagram for metastable helium atoms inthe triplet S (3 and triplet P (3p) states;

FIG. 3 is a graph of the second triplet ground state (2 8 Zeeman energyseparation (resonance frequency) of metastable helium atoms versusmagnetic field strengths; and

FIG. 4 is a graph of the relative spectral response of a presentlyavailable Boron-doped N on P silicon'photodetector as applied to themagnetometer of FIG. 1.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1,there is illustrated a metastable helium magnetometer of theself-oscillation type constructed in accordance with the presentinvention. Helium gas contained in a lamp 10 is ionized by an electricfield generated between electrodes 11 and 12 producing thereby light inthe complete helium spectrum including the particularemission linerequired for optically pumping metastable helium atoms according to theinvention. An ignition circuit 13 produces a high voltage a. c. signalwhich passes through an ignition filter l4 and a lamp matching network16 to provide a high voltage discharge at electrodes 11 and 12 necessaryto initiate ionization of the gas in the lamp 10. An excitor circuit 17thereafter maintains ionization with a suitable alternating current suchas 50 megacycles per second. For helium, approximately 1200 volts isnecessary initially to ionize the helium, but only a weak discharge fromthe excitor circuit 17 is required to maintain ionization. The lampmatching network 16 provides impedance matches at the input to the lamp10 for obtaining maximum power transfer.

' The light generated between electrodes 11 and 12 is collimated by lens18 and passed through interference filter 19 which suppresses light ofwavelengths other than 3888.65 Angstroms. The 3888.65 Angstrom light isthen linearly and circularly polarized by linear polarizer 21 andcircular polarizer 22 respectively and radiated on an absorption cell23. The cell 23 contains helium gas which is excited to the metastablesecond triplet ground state 2 S by an electric field produced byelectrodes 26 and 27. An absorption cell matching network 28 providesthe appropriate impedance matching for the electrical energy from eitherthe exciter circuit 17 or the ignition filter l4.

As shown in FIG. 2, the effect of 3888.65 Angstrom radiation in theabsorption cell 23 is to optically pump the meta-stable helium atomsfrom the 2 S state to the 3 P, state vis-avis the 2 F, state as occurswith 10,830 Angstrom radiation. In either case, the radiation energyapplied to the 2 S state of the metastable helium causes the atoms to beoptically pumped to the 2 F, or 3 nonmetastable state as the case maybe. These atoms return to the 2 S state in approximately. 10- to .l 0'seconds which is the lifetime of the 2 or 3-"P, states as compared to alifetime of 10' seconds for the 2S metastable state. In the presence ofa magnetic field, the 2S state experiences splitting into threesubstates (Zeeman energy levels) in which the energy separation betweensubstates is proportional to the field intensity. The energy levelseparation of the substates may be expressed as:

where:

f= Larmor frequency in Hertz H magnetic field strength in gammas, and

k gyromagnetic ratio for Helium of 28.024

Hertz/gammas. g

It is illustrated in FIG. 3, for example, that the Larmor frequency ofM, substate +1 will vary linearly from 0.7 X 10' to 2.1 X 10 Hertz withan ambient magnetic field change from 25,000 to 75,000 gammas. Themodulation frequency of the 3888.65 Angstrom light transmitted throughthe cell 23 will'correspond to the Larmor frequency because some of thelight has been absorbed in optically pumping the 2 S state atoms to the3 F, state. The linear polarization tends to overpopulate substates M,+1 and M, -1 at the expense of substate M, 0 during the transition backto the ZS The circular polarization, depending on whether it is rightorleft-hand, tends to overpopulate either the M, l or M, l substate at theexpense of the other substates. This overpopulation increases theamplitude of the modulating light transmitted through the cell 23.

The modulating light transmitted from the lamp is focused by focusinglens 33 onto a photo-detector whose output signal is modulated at thesame frequency as the transmitted light, and therefore corresponds tothe Larmor frequency at the ambient magnetic field intensity. FIG. 4shows the relative spectral response of the active material of thephoto-detector 34. It comprises a Boron-doped N on P silicon, such asmanufactured by Texas Instruments, Inc. as part number 531133-1. Of theemission lines suitable for optically pumping metastable Helium, thethree lines at approximately.3888.65 Angstroms produce the highestspectral response, all other emission lines producing relatively lowresponses. For example, the relative spectral response of thephoto-detector 34 at the 3888.65 Angstrom emission line is 42 percent ascompared to 13 percent for the next most responsive emission line of10,830 Angstroms. The present invention therefore utilizes these threelines which, as shown in FIG. 2 and noted above, optically pump themetastable helium atoms to the 3 state and produces an output signal ofhigh signal-to-noise ratio.

The photo-detector signal is amplified by preamplifier 36, fed to avariable phase amplifier 37, and finally to a pair of Helmholtz coils 31and 32 positioned 'to provide a uniform magnetic field in the cell 23trans-' verse to the direction of the light beam. The phase is adjustedin amplifier 37 so that, after the signal passes through the amplifier38, the phase of the signal applied to the resonance frequency coils 31and 32 will accelerate equalization in population of the atoms in all 2S substates thereby sustaining self-oscillation in the loop containingcomponents 31 through 38.

A display 39 connected to the output of variable phase amplifier 37senses the resonance frequency and presents it in a form indicative ofthe magnetic field intensity.

It will be understood that various changes in the details, materials,steps and arrangement of parts, which have been herein described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

What is claimed is:

1. A self-oscillating metastable helium magnetometer comprising:

light-means emitting light of 3888.65- Angstrom wavelength for opticallypumping metastable helium atoms to the 3 state;

1 quantum means receiving the light and absorbing the light at a rateproportional to the ambient magnetic field intensity;

measuring means optimally responsive to light at the emitted wavelengthtransmitted through said quantum means for producing an output signalindicative of the ambient magnetic field intensity; and

resonance means operatively connected to said quantum means and saidmeasuring means for maintaining self-oscillation.

2. A magnetometer according to claim 1 wherein said light meansincludes:

first circuit means for providing initial and sustaining output signalsof a predetermined voltage and frequency; emission means receiving thefirst circuit means output signal and emitting light in the heliumspectrum; and optical means receiving the light and producing a linearlyand circularly polarized collimated light beam of 3888.65 Angstromswavelength. 3. A magnetometer according to claim 2 wherein said opticalmeans includes:

a collimating lens receiving the emitted light and producing acollimated light beam; a light filter interposed in said light beamtransmitting therethrough light of only 3888.65 Angstroms wavelength;and polarizer means interposed in the filtered light beam for linearlyand circularly polarizing the light. 4. A magnetometer according toclaim 3 wherein said first circuit means includes: an ignition circuithaving a momentary high voltage output signal; an ignition filterreceiving the ignition circuit output signal and producing an initialhigh voltage output signal and producing a high voltage output signal ofa predetermined frequency bandwidth for initiating light emission; anexciter circuit providing a signal of frequency and voltage sufficientto sustain light emission after cessation of said ignition filter outputsignal; and a lamp matching network receiving the ignition filter andexciter circuit output signals and producing an impedance-matched outputsignal to said emission means. 5. A magnetometer according to claim 4wherein said quantum means includes:

a helium absorption cell interposed in the filtered and polarized lightbeam; and an absorption cell matching network receiving said filter andexciter circuit output signals and producing an impedance-matched outputsignal to said absorption cell or ionizing the helium therein. 6. Amagnetometer according to claim 5 wherein said measuring means includes:

a focusing lens receiving the light transmitted through said absorptioncell and focusing the light; a photo-detector optimally responsive tolight emission of 3888.65 Angstroms receiving the focused light andproducing an electrical output signal pro output signal and producing anamplified and phase adjusted output signal; and

a pair of Helmholtz resonance coils positioned about said absorptioncell receiving the second circuit means output signal and producing aresonance field in said absorption cell transverse to the light beam.

1. A self-oscillating metastable helium magnetometer comprising: lightmeans emitting light of 3888.65 Angstrom wavelength for opticallypumping metastable helium atoms to the 33PJ state; quantum meansreceiving the light and absorbing the light at a rate proportional tothe ambient magnetic field intensity; measuring means optimallyresponsive to light at the emitted wavelength transmitted through saidquantum means for producing an output signal indicative of the ambientmagnetic field intensity; and resonance means operatively connected tosaid quantum means and said measuring means for maintainingself-oscillation.
 2. A magnetometer according to claim 1 wherein saidlight means includes: first circuit means for providing initial andsustaining output signals of a predetermined voltage and frequency;emission means receiving the first circuit means output signal andemitting light in the helium spectrum; and optical means receiving thelight and producing a linearly and circularly polarized collimated lightbeam of 3888.65 Angstroms wavelength.
 3. A magnetometer according toclaim 2 wherein said optical means includes: a collimating lensreceiving tHe emitted light and producing a collimated light beam; alight filter interposed in said light beam transmitting therethroughlight of only 3888.65 Angstroms wavelength; and polarizer meansinterposed in the filtered light beam for linearly and circularlypolarizing the light.
 4. A magnetometer according to claim 3 whereinsaid first circuit means includes: an ignition circuit having amomentary high voltage output signal; an ignition filter receiving theignition circuit output signal and producing an initial high voltageoutput signal and producing a high voltage output signal of apredetermined frequency bandwidth for initiating light emission; anexciter circuit providing a signal of frequency and voltage sufficientto sustain light emission after cessation of said ignition filter outputsignal; and a lamp matching network receiving the ignition filter andexciter circuit output signals and producing an impedance-matched outputsignal to said emission means.
 5. A magnetometer according to claim 4wherein said quantum means includes: a helium absorption cell interposedin the filtered and polarized light beam; and an absorption cellmatching network receiving said filter and exciter circuit outputsignals and producing an impedance-matched output signal to saidabsorption cell or ionizing the helium therein.
 6. A magnetometeraccording to claim 5 wherein said measuring means includes: a focusinglens receiving the light transmitted through said absorption cell andfocusing the light; a photo-detector optimally responsive to lightemission of 3888.65 Angstroms receiving the focused light and producingan electrical output signal proportional to the intensity of the lighttransmitted through said absorption cell; and display means receiving anoutput signal from said second circuit means and producing an indicationof the ambient magnetic field intensity.
 7. A magnetometer according toclaim 6 wherein said resonance means includes: second circuit meansreceiving the photo-detector output signal and producing an amplifiedand phase adjusted output signal; and a pair of Helmholtz resonancecoils positioned about said absorption cell receiving the second circuitmeans output signal and producing a resonance field in said absorptioncell transverse to the light beam.